Inertia-compensated a.c. biased hydrophone incorporating a porous capacitance transducer

ABSTRACT

A porous capacitance transducer for producing electrical output signals in response to dynamic pressure fluctuations in a surrounding liquid medium substantially comprises an electrically conductive membrane in parallel spaced relation with a collimated hole structure (CHS) plate, which is a porous metal structure containing a controlled number and distribution of discrete parallel capillaries. An inertia and depth-compensated a.c. biased hydrophone incorporating the basic porous capacitance transducer substantially comprises first and second membranes respectively associated with first and second pairs of CHS plates and first and second pairs of oil-filled reservoirs. Such a hydrophone exhibits improved sensitivity, acoustic bandwidth response, dynamic pressure response, and noise immunity.

United States Patent Ammann INERTIA-COMPENSATED A.C. BIASED HYDROPHONEINCORPORATING A Primary Examiner-Benjamin A. Borchelt POROUS CAPACITANCEAssistant Examiner-Harold Tudor TRANSDUCER A norney--R. J. Guenther andKenneth B. Hamlin [75] Inventor: Hans Hugo Ammann, Chester, NJ. [57]ABSTRACT [73] Asslgnee: F gla g i ;???3? A porous capacitance transducerfor producing elecurrray er 6 ey elg trical output signals in responseto dynamic pressure [22] Filed: Mar. 25, 1971 fluctuations in asurrounding liquid medium substantially comprises an electricallyconductive membrane [21] Appl' lzsoll in parallel spaced relation with acollimated hole structure (C HS) plate, which is a porous metal struc-[52] [1.8. CI. ..340/8 LF, 340/12 ture containing a controlled numberand distribution [51] Int. Cl. ..H04h 13/00 of discrete parallelcapillaries. An inertia and depth- [58] Field of Search ..340/8, 8 L, 8LP, compensated a c, biased hydrophone incorporating 1 12 13 the basicporous capacitance transducer substantially comprises first and secondmembranes respectively as [56] References Cited sociated with first andsecond pairs of CBS plates and first and second pairs of oil-filledreservoirs. Such a UNITED STATES, PATENTS hydrophone exhibits improvedsensitivity, acoustic 2,545,101 3/1951 Meunier ..340/8 LF bandwidthresponse, dynamic pressure response, and 3,560,913 2/1971 Copley noiseimmunity. 3,403,375 9/1968 Wright, Jr. et al. 1,471,547 10/1923Chilowsky et a1 ..340/8 LP 13 Claims, 3 Drawing Figures i 5 2 51 l 3'5:551 2; 24@- 24b Zi 3 -23 12 43 1 I 35 11 T. 47 f 46 f" X 30a I], ll 30b50 53 52 INERTllA-COMPENSATED A.C. BIASED HYDROPIIONE INCORPORATING APOROUS CAPACITANCE TRANSDUCER GOVERNMENT CONTRACT The invention hereinclaimed was made in the course of or under a contract withthe'Department of the Navy.

FIELD OF THE INVENTION This invention relates to hydrophones and inparticular to an inertia and depth-compensated a.c. biased hydrophoneincorporating a porous capacitance transducer.

BACKGROUND OF THE INVENTION inertia-compensation in which case thehydrophone I output signals result from dynamic pressure fluctuationsand not from longitudinal hydrophone acceleration. Up to nowcompensation for signal interferences, such as longitudinal hydrophoneacceleration, has been found impracticable in many cases.

It is therefore an object of this invention to provide adepth-compensated hydrophone incorporating a porous capacitancetransducer.

It is another object of this invention to provide an inertia-compensatedhydrophone incorporating a porous capacitance transducer.

SUMMARY OF THE INVENTION According to the present invention, a porouscapacitance transducer for producing electrical output signals inresponse to dynamic pressure fluctuations in a surrounding liquidmediumsubstantially comprises an electrically conductive membrane inparallel spaced relation with a collimated hole structure (Cl-IS) plate,which is a porous metal structure containing a controlled number anddistribution of discrete parallel capillaries.

According to the present invention, an inertia and depth-compensateda.c. biased hydrophone incorporating the porous capacitance transducersubstantially comprises first and second membranes respectivelyassociated with first and second pairs of CBS plates and first andsecond pairs of oil-tilled reservoirs.

It is an advantage of this invention that the hydrophone is inexpensiveto manufacture, easy to assemble and disassemble, and physically small.

It is another advantage of this invention that the hydrophone exhibitsimproved sensitivity, acoustic bandwidth response, dynamic pressureresponse, and noise immunity.

It is a feature of this invention that the CHS plate has mechanicalstrength substantially equal to that of a solid plate.

It is another feature of this invention that the CH8 plate provides aflow area approximately equal to 41 percent of the total area of a solidplate.

It is a further feature of this invention that the CH8 plate provides acapacitance substantially equal (i.e., 92 percent) to that of a solidplate of the same external dimensions.

It is a still further feature of this invention that in the proposedhydrophone no net forces due to electric fields are imparted to themembranes.

It is a still further feature of this invention that in the proposedhydrophone substantially no thermal noise results since purely reactiveelements are utilized.

DESCRIPTION OF THE DRAWING The above and other objects, advantages, andfeatures of this invention will be better appreciated by a considerationof the following detailed description and the drawing in which:

FIG. 1 is a cross section through a diameter of a porous capacitancetransducer according to the present invention;

FIG. 2 is a longitudinal sectional view of a hydrophone incorporatingthe porous capacitance transducer of the present invention; and

FIG. 3 is a circuit diagram showing four variable a capacitancemembrane-CH5 plate pairs in a normally balanced bridge circuit which isdriven by an altemating current source.

DETAILED DESCRIPTION Referring to FIG. 1, porous capacitance transducer10 comprises electrically conductive circular membrane 11 in parallelspaced relation with collimated hole structure (CI-IS) plate 12, whichis a porous metal structure, of the type manufactured, for example, bythe Brunswick Corporation of Chicago, Illinois, containing a controllednumber and distribution of discrete parallel capillaries l3. Collimatedhole structure plates are described in a Development Report of theTechnical Products Division (Form 4-001 Brunswick Corporation, copyrightl968. Membrane l1 and plate 12 which are electrically insulated fromeach other,

form the elements of a variable capacitor. Surface 14 of plate 12, isspherically concave in order to improve the sensitivity and noiseimmunity of transducer 10. Further, transducer 10 is surrounded byliquid medium 15, part of which fills gap 16 located between membrane l1and plate 12. Membrane 11 can advantageously be made of aluminum or be asandwich of metallized mylar.

A desirable feature of a hydrophone is automatic depth compensation.This, of course, allows ambient or static pressure equalization withinthe hydrophone structure without simultaneous dynamic pressureequalization. In such a case, acoustic sensitivity is assured and staticequilibrium maintained. It is apparent that transducer 10, to someextent, has the automatic depth-compensation feature since parallelcapillaries l3 allow for ambient pressure equalization within gap 16.However, any sudden or dynamic pressure fluctuations in liquid medium 15are attenuated before reaching membrane 11 via the capillary route. Inother words, parallel capillaries 13 act as restrictive orifices whichblock any dynamic pressure fluctuations from being applied to membrane11 from the right. Therefore, a dynamic pressure fluctuation insurrounding liquid 15 causes a deflection of membrane 11 relative toplate 12 thereby effecting a detectable change of capacitance.

It has been determined that CHS plate 12 has mechanical strengthsubstantially equal to that of a solid plate, provides a flow areaapproximately equal to 41 percent of the total area of a solid plate,and provides a capacitance substantially equal (i.e., 92 percent) tothat of a solid plate.

FIG. 2 is a longitudinal sectional view of an inertia anddepth-compensated hydrophone incorporating porous capacitance transducer10 of FIG. 1. Hydrophone comprises hollow cylindrical steel housing 21further including threaded hole 22 to which eye bolt 80 is fastened.Attached to eye bolt 80 is an undersea towing cable, not shown, whichcontains a plurality of electrical conductors. Housing 21 also includeswall 23 which divides the housing into two separate but similarchambers.

Hydrophone 20 further comprises transducer structure 35 which includesCHS plates 36 and 37, membrane 38, and membrane mounting ring 39, andtransducer structure 45 which includes CHS plates 46 and 47, membrane48, and membrane mounting ring 49. Circular membranes 38 and 48 aresupported at their periphery by membrane mounting rings 39 and 49,respectively. The CBS plate pairs form double capacitors with theirassociated membranes acting as common elements. The housing, the CBSplates, and the membranes are electrically insulated from each other, bymeans not shown.

Advantageously, the membrane mounting rings are made of steel while themembranes are made of aluminum. Fastening of a membrane to itsassociated ring is done at a slightly elevated temperature, such as 100F. Upon cooling of the members, differential thermal contractionproduces the desired nominal membrane tension.

Housing 21 further includes annular shoulders 24a and 24b and threadedportions a and 25b. Thus, transducer structures 35 and 45 are secured tohousing 21 by threading annular retaining rings 31a and 31b ontoportions 25a and 25b, respectively, whereupon the transducer structuresare caused to bear against their respective shoulders.

I-Iydrophone 20 further comprises boot structures 30a and 30b ofyieldable sound transmitting material which cover the open ends ofhousing 21. The bonding or sealing of boots 30a and 30b relative tohousing 21 is done using well-known techniques. Boot 30a and membrane 38define depth-compensating reservoir while membrane 38 and wall 23 definepressure-release reservoir 51. In a similar manner, boot 30b andmembrane 48 define depth-compensating reservoir 52 while membrane as andwall 23 define pressure-release reservoir 53. It should be noted thatreservoirs 50 and 51 are respectively similar to reservoirs 52 and 53.Reservoirs 50, 51, 52 and 53 contain a liquid, such as oil, having acompressibility exceeding that of the surrounding liquid medium, waterin this case, in which the hydrophone is to be immersed.

Automatic depth-compensation of hydrophone 20 is effected byflow-communicating means such as restricted orifice in membrane 38 andcapillary hole 86 in housing 21. It is apparent that orifice 85 allowslimited flow between reservoirs 50 and 51 while hole 86 allows limitedflow between reservoirs 52 and 53. Hole 86 may be machined on housing 21using well known techniques such as by longitudinal as well as radialdrilling and by utilizing threaded plug 87. The method ofdepth-compensation used is determined by the particular application.

Boots 30a and 30b, which are exposed to the surrounding liquid medium,expand and contract in order to equalize the oil pressure withinreservoirs 50, 51, 52, and 53 with the ambient hydrostatic pressure.Such static pressure equalization between reservoirs 50 and 51 andbetween reservoirs 52 and 53 is effected by means of restrictive orifice85 and capillary hole 86, respectively. It should be noted that the oilvolume within the depth-compensating reservoirs must be sufficient toenable ample fluid flow to their associated pressure-release reservoirsfor the full range of ambient hydrostatic pressures expected. However,dynamic pressure fluctuations, which are transmitted by boots 311a and30b to their respective depth-compensating reservoirs, do not reach thepressure-release reservoirs, since such fluctuations are attenuated byrestrictive orifice 85 and capillary hole 86.

Therefore, the oil in reservoirs 50, 51, 52, and 53, together withrestrictive orifice 85 and capillary hole 86, decouple membranes 38 and48 from the high ambient hydrostatic pressure while simultaneouslyallowing them to sense dynamic pressure fluctuations. In addition tocompensating for such high ambient hydrostatic pressure, the oil withinpressure-release reservoirs 51 and 53 places an equivalent springstiffness in parallel with the effective membrane stiffness. While theoil is relatively compressible, the stiffness of the oil withinreservoirs S1 and 53 may be much greater than the effective membranestiffness thereby substantially influencing the sensitivity, acousticbandwidth response, and dynamic range of hydrophone 20. The dynamicrange is given when there is contact between the membrane and the CH8plate. Therefore, the desired nominal membrane tension can be relativelylow since such tension is called upon only to establish the membranesnull position. The low membrane tension requirement greatly simplifiesthe fabrication and mounting of membranes 38 and 48 on mounting rings 39and 49, respectively.

Housing 21 can be characterized as comprising left and rightback-to-back cup-shaped structures whose respective open ends areoppositely directed and whose respective closed ends are formed bycommon wall 23. As mentioned above, the open ends of these left andright cup-shaped structures are respectively covered by boots 30a and30b. Depth-compensating and pressurerelease reservoirs, as definedabove, are respectively associated with these left and right cup-shapedportions of housing 21. In light of the above, it is apparent that thosecomponents of hydrophone 20 to the left and right of wall 23respectively form first and second depth-compensated hydrophonestructures. The combination of these two hydrophone structures yieldsthe inertia and depth-compensated hydrophone, as will become moreapparent hereinafter.

Referring back to FIG. 2, the output e of a.c. source 60 is applied tomembranes 38 and 48 by leads 61 and 62, respectively. Leads 61 and 62are attached to their respective membranes by brazing or other suitablemethod. In addition, CI-IS plates 36, 37, 46, and 47 are connectedelectrically to the input terminals of detection means 70 by leads 71,72, 73, and 74, respectively. It should be recalled that the two pairsof CHS plates form two double capacitances with their associatedmembranes acting as common elements. It should also be noted that leads61, 62, 71, 72, 73 and 74 are part of the undersea towing cable, notshown, which is attached to eye bolt 80.

FIG. 3 is a circuit diagram showing four variable capacitances C C C andC in a normally balanced bridge 90 which is driven by a.c. source 60 vialeads 61 and 62. Capacitance C which is located between bridge terminals75 and 78, comprises membrane 48 and CH8 plate 46; capacitance C whichis located between bridge terminals 75 and 76, comprises membrane 48 andCH8 plate 47; capacitance C which is located between bridge terminals 76and 77, comprises membrane 38 and CH8 plate 36; and capacitance C whichis located between bridge terminals 77 and 78, comprises membrane 38 andCBS plate 37. With the capacitances arranged in the bridge as shown, nonet forces due to electric fields are imparted to the membranes in whichcase the electrical circuit does not influence the mechanical responseof the membranes. Output e, of bridge 90 is detected across terminals 76and 78 by detection means 70. It is assumed that detection means 70 hasinfinite input impedance.

In light of the above,-the voltage across capacitance where Z and Z, arethe impedances across capacitors C and C.,, respectively, and e, is theinput a.c. voltage from source 60. Similarly, the voltage acrosscapacitance C can be written as:

2 a/( 2 a) i Therefore, the voltage at terminal 78 is given by:

while the voltage at terminal 76 is given by:

1s t a)/( 2+ s) l- Taking the difference of e and e yields the outputvoltage e, which is given by:

s/ 2+ a) 4/ 1+ 4) d Recalling that hydrophone has the depth-compensation feature, a change in the ambient hydrostatic pressure yields nooutput e since in such a case C, C C, C,

Now, in order to show that hydrophone 20 is inertia compensated, i.e.,that the bridge output e, is not affected by acceleration of hydrophone20 along its longitudinal axis, it is assumed that capacitances C C Cand C have the nominal value C and that they change by the amount 6.Therefore, if hydrophone 20 is accelerated to the right, C becomes C e,C becomes C, 6, C becomes C e, and C becomes C e as a result of theinertia of the oil in depth-compensating reservoir 52 andpressure-release reservoir 51. Substituting these values into equation(5) yields:

A similar result is gotten when hydrophone 20 is accelerated to theleft. However, if hydrophone 20 is subjected to a dynamic pressurefluctuation, C becomes C, e, C becomes C e, C becomes C, e, and C,becomes C 5. Substituting these values into equation (5) yields:

o= o t=(/ t- (9) Therefore, hydrophone 20 is sensitive to dynamicpressure fluctuations but insensitive to longitudinal acceleration. Inthe above cases, it is assumed that the distance between membranes 38and 48 is small compared to the wavelength of the acoustic signal to bedetected.

In summary, transducer 10 of FIG. 1 is particularly useful in hydrophone20 since the diameter of membrane 11 and CH8 plate 12 is substantiallyequal to the inner diameter of housing 21. This, of course, lends itselfto a compact hydrophone structure. Since hydrophone 20 comprisesrelatively few parts, it is inexpensive to manufacture and easy toassemble and disassemble. Also, very little thermal noise results sincepurely reactive elements are utilized. Additional advantages accrue inthe area of signal processing since the carrier is modulated directlythereby reducing multiplexing requirements. Also, the l/f noisecontribution is greatly reduced and simpler, more compact a.c.amplifiers can be employed.

It will be recalled that CBS plate 12 of FIG. ll has spherically concavesurface 14. It is shown by H. V. P. Neubert in Instrument Transducers,Oxford, 1963, that when the moving element in a capacitance transduceris a thin membrane, the change in capacitance, e, divided by the nominalcapacitance C,,, i.e., e/C is only half that obtained when a rigid plateis moved an amount equal to the maximum membrane displacement. However,it can also be shown that e/C for the case where a rigid plate isdisplaced a certain fraction of its quiescent gap, is the same as e/Cfor the case where a membrane is displaced the same fraction of itsquiescent gap relative to a concave surface. This is true if the shapeof the backing plate's surface is similar to surfaces can be ground withutmost precision, very small gaps between the diaphragm and the CH8plate are possible. Therefore, e/C for the membrane with the sphericallyconcave CHS plate is comparable to that of a flat plate of equal area.In addition, for the spherically concave CHS plate, C is much greaterfor equal surface area since smaller quiescent gaps are possible. Thenominal capacitance C, should be large to minimize the effect of straycapacitance on sensitivity. Therefore, the effective e/C, for themembrane with the spherically concave CHS plate results in highersensitivity and greater noise immunity since both e/C, and C, areincreased.

While the arrangement according to this invention for detecting dynamicpressure fluctuations in a surrounding liquid medium has been describedin terms of a specific embodiment, it will be apparent to one skilled inthe art that many modifications are possible within the spirit and scopeof the disclosed principle.

What is claimed is:

l. A porous capacitance transducer responsive to dynamic pressurefluctuations and substantially nonresponsive to static pressurefluctuations in a surrounding liquid medium comprising an electricallyconductive membrane subjected to mechanical excitation by said dynamicpressure fluctuations, and

a collimated hole structure plate immersed in said surrounding liquidmedium and in parallel spaced relation with said membrane,

said membrane, surrounding liquid medium, and plate having a capacitancesubstantially equal to that of an uncollimated hole structure plate ofthe same overall area, and

said plate impeding the transfer of said surrounding liquid mediumtherethrough when said transducer is subjected to dynamic pressurefluctuations and providing substantially unimpeded transfer of saidsurrounding liquid medium therethrough when said transducer is subjectedto static pressure fluctuations.

2. The porous capacitance transducer of claim 1 wherein the platesurface adjacent said membrane is concave.

3. The porous capacitance transducer of claim 2 wherein said concavesurface is spherical.

4. A depth-compensated hydrophone responsive to dynamic pressurefluctuations and substantially nonresponsive to static pressurefluctuations in a surrounding liquid medium, said hydrophone comprising:

a cylindrical housing, said housing further including a cup-shapedstructure having one open end and one closed end;

a boot structure of yieldable sound transmitting material covering saidopen end and intimately attached to said housing;

a porous capacitance transducer including an electrically conductivemembrane and first and second collimated hole structure plates, saidfirst and second plates respectively being located on opposite sides ofsaid membrane and in parallel spaced relation therewith;

means for securing said transducer within said housing, said bootstructure and said transducer defining a depth-compensating reservoirand said transducer and said closed end defining a pressurereleasereservoir;

a pressure transmitting liquid filling said reservoir and having acompressibility exceeding that of said surrounding liquid medium; and

flow-communicating means joining said reservoirs for impeding thetransfer of said pressure transmitting liquid between said reservoirswhen said hydrophone is subjected to dynamic pressure fluctuations andfor providing substantially unimpeded transfer of said pressuretransmitting liquid between said reservoirs when. said hydrophone issubjected to static pressure fluctuations.

5. The hydrophone of claim 4 wherein the plate surfaces adjacent saidmembrane are concave.

6. The hydrophone of claim 5 wherein said concave surfaces arespherical.

7, The hydrophone of claim 4 wherein said flowcommunicating means is arestrictive orifice in said membrane.

8. The hydrophone of claim 4 wherein said flowcommunicating means is arelatively small diameter orifice in said housing.

9. An inertia and depth-compensated hydrophone responsive to dynamicpressure fluctuations in a surrounding liquid medium and substantiallynonresponsive to both hydrostatic pressure fluctuations in said mediumand to longitudinal hydrophone acceleration comprising a cylindricalhousing including a wall which divides said housing into first andsecond open-end chambers,

first and second boot structures of yieldable sound transmittingmaterial respectively covering said first and second open-end chambersand intimately attached to said housing,

first and second porous capacitance transducers each including anelectrically conductive membrane and two collimated hole structureplates one on each side of said membrane and in parallel spaced relationtherewith,

first and second means for respectively mounting said first and secondtransducers within said first and second chambers whereby said firstboot structure and said first transducer define a firstdepth-compensating reservoir, said transducer and said wall define afirst pressure-release reservoir, said second boot structure and saidsecond transducer define a second depth-compensating reservoir, and saidsecond transducer and said wall define a second pressure-releasereservoir,

a pressure transmitting liquid having a compressibility exceeding thatof said surrounding liquid medium filling said reservoirs, and

first and second flow-communicating means respectively joining thedepth-compensating reservoirs with their associated pressure-releasereservoirs for impeding the transfer of said pressure transmittingliquid between said associated reservoirs when said hydrophone issubjected to dynamic pressure fluctuations and for providingsubstantially unimpeded transfer of said pressure transmitting liquidbetween said associated reservoirs when said hydrophone is subjected tostatic pressure fluctuations.

10. The hydrophone of claim 9 wherein the plate sursecondflow-communicating means are restrictive orifices in their respectivemembranes.

13. The hydrophone of claim 9 wherein said first and secondflow-communicating means are relatively small diameter orifices in saidhousing.

1. A porous capacitance transducer responsive to dynamic pressurefluctuations and substantially non-responsive to static pressurefluctuations in a surrounding liquid medium comprising an electricallyconductive membrane subjected to mechanical excitation by said dynamicpressure fluctuations, and a collimated hole structure plate immersed insaid surrounding liquid medium and in parallel spaced relation with saidmembrane, said membrane, surrounding liquid medium, and plate having acapacitance substantially equal to that of an uncollimated holestructure plate of the same overall area, and said plate impeding thetransfer of said surrounding liquid medium therethrough when saidtransducer is subjected to dynamic pressure fluctuations and providingsubstantially unimpeded transfer of said surrounding liquid mediumtherethrough when said transducer is subjected to static pressurefluctuations.
 2. The porous capacitance transducer of claim 1 whereinthe plate surface adjacent said membrane is concave.
 3. The porouscapacitance transducer of claim 2 wherein said concave surface isspherical.
 4. A depth-compensated hydrophone responsive to dynamicpressure fluctuations and substantially nonresponsive to static pressurefluctuations in a surrounding liquid medium, said hydrophone comprising:a cylindrical housing, said housing further including a cup-shapedstructure having one open end and one closed end; a boot structure ofyieldable sound transmitting material covering said open end andintimately attached to said housing; a porous capacitance transducerincluding an electrically conductive membrane and first and secondcollimated hole structure plates, said first and second platesrespectively being located on opposite sides of said membrane and inparallel spaced relation therewith; means for securing said transducerwithin said housing, said boot structure and said transducer defining adepth-compensating reservoir and said transducer and said closed enddefining a pressure-release reservoir; a pressure transmitting liquidfilling said reservoir and having a compressibility exceeding that ofsaid surrounding liquid medium; and flow-communicating means joiningsaid reservoirs for impeding the transfer of said pressure transmittingliquid between said reservoirs when said hydrophone is subjected todynamic pressure fluctuations and for providing substantially unimpededtransfer of said pressure transmitting liquid between said reservoirswhen said hydrophone is subjected to static pressure fluctuations. 5.The hydrophone of claim 4 wherein the plate surfaces adjacent saidmembrane are concave.
 6. The hydrophone of claim 5 wherein said concavesurfaces are spherical.
 7. The hydrophone of claim 4 wherein saidflow-communicating means is a restrictive orifice in said membrane. 8.The hydrophone of claim 4 wherein said flow-communicating means is arelatively small diameter orifice in said housing.
 9. An inertia anddepth-compensated hydrophone responsive to dynamic pressure fluctuationsin a surrounding liquid medium and substantially nonresponsive to bothhydrostatic pressure fluctuations in said medium and to longitudinalhydrophone acceleration comprising a cylindrical housing including awall which divides said housing into first and second open-end chambers,first and second boot structures of yieldable sound transmittingmaterial respectively covering said first and second open-end chambersand intimately attached to said housing, first and second porouscapacitance transducers each including an electrically conductivemembrane and two collimated hole structure plates one on each side ofsaid membrane and in parallel spaced relation therewith, first andsecond means for respectively mounting said first and second transducerswithin said first and second chambers whereby said first boot structureand said first transducer define a first depth-compensating reservoir,said transducer and said wall define a first pressure-release reservoir,said second boot structure and said second transducer define a seconddepth-compensating reservoir, and said second transducer and said walldefine a second pressure-release reservoir, a pressure transmittingliquid having a compressibility exceeding that of said surroundingliquid medium filling said reservoirs, and first and secondflow-communicating means respectively joining the depth-compensatingreservoirs with their associated pressure-release reservoirs forimpeding the transfer of said pressure transmitting liquid between saidassociated reservoirs when said hydrophone is subjected to dynamicpressure fluctuations and for providing substantially unimpeded transferof said pressure transmitting liquid between said associated reservoirswhen said hydrophone is subjected to static pressure fluctuations. 10.The hydrophone of claim 9 wherein the plate surfaces adjacent theirassociated membranes are concave.
 11. The hydrophone of claim 10 whereinsaid concave surfaces are spherical.
 12. The hydrophone of claim 9wherein said first and second flow-communicating means are restrictiveorifices in their respective membranes.
 13. The hydrophone of claim 9wherein said first and second flow-communicating means are relativelysmall diameter orifices in said housing.